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Optical absorption in glow discharge amorphous silicon films
Thin Solid Films, 58 (1979)397-401 0 Elsevier Sequoia !%A., Lausanne-Printed
OPTICAL ABSORPTION FILMS*
397
in the Netherlands
IN GLOW DISCHARGE
AMORPHOUS
SILICON
J. MCGILL AND J. I. B. WILSON Department of Physics, Heriot- Watt University, Riccarton, Edinburgh (Gt. Britain) (Received August 9, 1978; accepted September 15,197s)
Both undoped films and films doped with phosphine were deposited by the r.f. glow discharge decomposition of silane onto 7059 glass and single-crystal silicon wafers. The absorption edge in the near infrared was examined and the refractive index was calculated from transmittance interference fringes. The various siliconhydrogen infrared absorption bands were examined to see whether hydrogen incorporation varied with the growth conditions. Our films invariably showed the presence of secondary and tertiary bonded hydrogen. 1. INTRODUCTION
As part of our study of thin film amorphous silicon solar cells’*2 we investigated the optical absorption of the semiconductor with the aim of characterizing film quality by the IR absorption bands. In addition, the optical constants of these films are required for the design of effective antireflection coatings. A set of films 1-3 urn thick were specially prepared on both 7059 glass and single-crystal silicon wafers using growth conditions thought to be close to the optimum for efficient cells. 2. FILM GROWTH In the apparatus employed for the r.f. glow discharge decomposition of silane we used a coil around the fused silica reaction chamber for coupling to the gases and to a water-cooled substrate holder, as described, previously3-5. A low pressure maintained by a rotary pump was measured by a Pirani gauge below the deposition chamber. Argon was used to purge the regulators, flowmeters, gas lines and pump. Both the substrate position within the discharge and the reaction chamber geometry are important for the maintenance of a stable plasma. The substrate temperature (180-320°C) and the gas pressure (0.1-0.5 Torr) were varied to produce this set of films, but the r.f. power input was held constant at some tens of watts. The deposition rate was below 10 A s-l. The undoped films had dark resistivities of lo’-lo6 R cm and the doped films had dark resistivities of around 10’ 0 cm. 3. REFRACTIVE INDEX AND ABSORPTION EDGE From the transmittance
interference
l Paper presented at the Fourth International September 1l-15, 1978; Paper 553.
fringes of the films in the near IR,
Congress on Thin Films, Loughborough,
Gt. Britain,
398
I. MCGILL, J. I. B. WILSON
corrected for reflectance loss, the refractive indices n, and film thicknesses d were calculated according to the following equation@ :
TInSX (C -n,2)(1 -Q) -= l-t 2n,2(1 +n,2)
Tmin
where n, is the substrate refractive index, and T,,,,, and Tmi, are the transmittance maxima and minima. d=-----1
124
(2)
2n, i2-L1
where I, and I, are wavelengths for adjacent maxima. Figure 1 shows the real part of the refractive index decreasing to a constant value at long wavelengths. It was almost independent of substrate temperature. (The imaginary part of the refractive index is close to zero in this region.) Figure 2 shows the absorption edge (assuming parabolic band edges) of films prepared at different substrate temperatures and indicates a shift towards a higher band gap for decreasing T,,, in agreement with results on films prepared elsewhere” *. t
I t
I
0
1
wavelength J.I~~
3
IO
t3V
2.0
Fig. 1. The refractive index of r.f. glow discharge amorphous silicon films. Fig. 2. The absorption edge of glow discharge amorphous silicon films prepared at the substrate temperatures T (“C) shown. For clarity only the lower points on each plot are shown.
OPTICAL ABSORPTION IN GLOW DISCHARGE AMORPHOUS
Si FILMS
399
4. INFRARED ABSORPTION BANDS Glow discharge amorphous silicon contains several atomic per cent of hydrogen by virtue of its growth conditions. This improves the effectiveness of normal substitutional doping in controlling the electrical conductivity by passivating dangling silicon bonds. If high silane pressures and low substrate temperatures are selected, then it is possible to produce films closer to a silane polymer (SiH,), than to amorphous silicon containing hydrogen3sg. The less cohesive higher polymer films are not suitable for solid state devices. The IR absorption spectrum from 3 to 20 urn contains evidence of Si-H, Si-Si and possible contaminant groups. The magnitude of each absorption band can be used to estimate the numbers of each bond type in the sample. Brodsky et aL” have assigned the bands at approximately 2000-2100 cm-’ to SiH, stretch modes, those at 850-890 cm- 1 to SiH, bending (“deformation”) modes and that at 640 cm- ’ to SiH, wagging modes. From these it is possible to see how much of the hydrogen is incorporated as SiH, or SiH, rather than as SiH. In particular, the bending mode is absent for films with only SiH bonds. A typical survey IR spectrum from a Perkin-Elmer 580 double beam spectrophotometer, with an uncoated silicon substrate as reference, is shown in Fig. 3 for a film on crystalline silicon.
4000
3000 wavenumber
2000 cm”
1000
Fig. 3. The IR transmittance of an amorphous silicon film on a single-crystal silicon substrate (substrate temperature, 186 “C; Pirani gauge reading, 0.3 Torr). Each absorption band has detailed structure according to the number of hydrogen atoms bonded to each silicon atom.
The simple SiH stretching mode at 2000 cm-’ (ols) was only seen for high substrate temperature films immediately after their preparation. All films absorbed
400
J. MCGILL, J. I. B. WILSON
more strongly at the wavelengths assigned to SiH, and SiH, (a,‘, w3’) but the magnitude of the stretch mode absorption band decreased at the higher substrate temperatures (Fig. 4). The absorption coefficient for film 145 at 2090 cm-’ was films had much weaker approximately 1500 cm- l. The phosphine-doped absorption in this band than was expected from a comparison with undoped films prepared under similar conditions. The small band at 2265 cm-‘, apparent on all films, has not been identified. Figure 5 shows the bending mode absorption which was present for all our films. It was absent from high temperature IBM” and RCA7 films. Clear resolution into two bands (as shown for 147) occurred only for the lower temperature films, and this changed after a few days to a single broad band (as shown for 143). This may suggest growth of the SiH, groups at the expense of SiH,. Sample 145 had an absorption coefficient of approximately 1640 cm-r at 880 cm-‘.
147 146
800
1600 wavenumber
cm-’
900 wavenumber
600 -1
cm
Fig. 4. The SiH, stretch mode absorption band for films prepared at the substrate temperatures T ((C) shown and at a pressure of0.2-0.3 Torr. (The wavenumber resolution is better than 3 cm- ‘.) Samples 141 and 143 were doped. Fig. 5. The SiH, bending mode absorption resolution is better than 4 cm- I.)
band for the same films as in Fig. 4. (The wavenumber
OPTICAL
ABSORPTION
IN GLOW DISCHARGE
AMORPHOUS
Si FILMS
401
The strong wagging mode absorption at 640 cm-’ was weakest in the higher substrate temperature films and in the doped films. From assignments of the detailed structure of this band” our films again showed stronger absorption by SiH, and SiH, than by SiH. Only slight evidence of the 590 cm- 1 rocking mode was seen in all films. Aging of the films by’vacuum heating for 1 h at 200 “C (i.e. below the deposition temperature) produced a small increase in the bend and wagging mode absorptions but no change in the other absorption band. Zanzucchi et al.’ observed a similar apparent increase in hydrogen content after brief vacuum annealing. This part of the study is continuing and we have no explanation for the observations as yet. From the magnitude of the absorption bands we estimated that our films contained 3 x 1O22cm-3 Si-H bonds. 5.
CONCLUSIONS
The differences in geometry, r.f. power input and unidentified parameters between various glow discharge chambers make it difficult to define uniquely the gas pressure and substrate temperature required to produce good quality amorphous Si: H films. Although it was expected that temperatures around 250 “C and silane pressures around 0.1 Torr would produce films containing mainly Si-H, our apparatus gave films with a large proportion of SiH, and SiH, as evidenced by the presence of the bending mode absorption. The shape of the SiH, stretch mode was affected by the growth conditions and by heavy doping with phosphine. We have extended our study to correlate solar cell performance with film structure and we have already found that good photovoltaic diodes can be fabricated on our material. ACKNOWLEDGMENTS
We wish to thank Fiona Riddoch and Alec Wallace for assistance in film preparation. We are grateful to the SRC and the CEC for financial support of the project. REFERENCES
1 2 3 4 5 6 7 8 9 10
J. I. B. Wilson, J. McGill and S. Kinmond, Nature (London), 272(1978) 152-153. J. I. B. Wilson and J. McGill, Solid-State Electron Devices, 2 (1978) S7-SIO. M. H. Brodsky, Thin Solid Films, 40 (1977) L23-L25. R. C. Chittick, J. H. Alexander and H. F. Sterling, J. Electrochem. Sot., 116 (1969) 77. W. E. Spear and P. G. LeComber, Philos. Mug., 33 (1976) 935-949. J. Soohoo and R. D. Henry, J. Appl. Phys., 49 (1978) 801-803. P. J. Zanzucchi, C. R. Wronski and D. E. Carlson, J. Appl. Phys., 48 (1977) 5227-5236. R. J. Loveland, W. E. Spear and A. Al-Sharbaty, J. Non-Cryst. Solids, I3 (1973174) 55-68. M. H. Brodsky and M. Cardona, Local order as determined by electronic and vibrational spectroscopy: amorphous semiconductors, J. Non-Cryst. So/ids, 31(1978) 81-108. M. H. Brodsky, M. Cardona and J. J. Cuomo, Phys. Rev., Sect. B, I6 (1977) 3556-3571.
OPTICAL ABSORPTION FILMS*
397
in the Netherlands
IN GLOW DISCHARGE
AMORPHOUS
SILICON
J. MCGILL AND J. I. B. WILSON Department of Physics, Heriot- Watt University, Riccarton, Edinburgh (Gt. Britain) (Received August 9, 1978; accepted September 15,197s)
Both undoped films and films doped with phosphine were deposited by the r.f. glow discharge decomposition of silane onto 7059 glass and single-crystal silicon wafers. The absorption edge in the near infrared was examined and the refractive index was calculated from transmittance interference fringes. The various siliconhydrogen infrared absorption bands were examined to see whether hydrogen incorporation varied with the growth conditions. Our films invariably showed the presence of secondary and tertiary bonded hydrogen. 1. INTRODUCTION
As part of our study of thin film amorphous silicon solar cells’*2 we investigated the optical absorption of the semiconductor with the aim of characterizing film quality by the IR absorption bands. In addition, the optical constants of these films are required for the design of effective antireflection coatings. A set of films 1-3 urn thick were specially prepared on both 7059 glass and single-crystal silicon wafers using growth conditions thought to be close to the optimum for efficient cells. 2. FILM GROWTH In the apparatus employed for the r.f. glow discharge decomposition of silane we used a coil around the fused silica reaction chamber for coupling to the gases and to a water-cooled substrate holder, as described, previously3-5. A low pressure maintained by a rotary pump was measured by a Pirani gauge below the deposition chamber. Argon was used to purge the regulators, flowmeters, gas lines and pump. Both the substrate position within the discharge and the reaction chamber geometry are important for the maintenance of a stable plasma. The substrate temperature (180-320°C) and the gas pressure (0.1-0.5 Torr) were varied to produce this set of films, but the r.f. power input was held constant at some tens of watts. The deposition rate was below 10 A s-l. The undoped films had dark resistivities of lo’-lo6 R cm and the doped films had dark resistivities of around 10’ 0 cm. 3. REFRACTIVE INDEX AND ABSORPTION EDGE From the transmittance
interference
l Paper presented at the Fourth International September 1l-15, 1978; Paper 553.
fringes of the films in the near IR,
Congress on Thin Films, Loughborough,
Gt. Britain,
398
I. MCGILL, J. I. B. WILSON
corrected for reflectance loss, the refractive indices n, and film thicknesses d were calculated according to the following equation@ :
TInSX (C -n,2)(1 -Q) -= l-t 2n,2(1 +n,2)
Tmin
where n, is the substrate refractive index, and T,,,,, and Tmi, are the transmittance maxima and minima. d=-----1
124
(2)
2n, i2-L1
where I, and I, are wavelengths for adjacent maxima. Figure 1 shows the real part of the refractive index decreasing to a constant value at long wavelengths. It was almost independent of substrate temperature. (The imaginary part of the refractive index is close to zero in this region.) Figure 2 shows the absorption edge (assuming parabolic band edges) of films prepared at different substrate temperatures and indicates a shift towards a higher band gap for decreasing T,,, in agreement with results on films prepared elsewhere” *. t
I t
I
0
1
wavelength J.I~~
3
IO
t3V
2.0
Fig. 1. The refractive index of r.f. glow discharge amorphous silicon films. Fig. 2. The absorption edge of glow discharge amorphous silicon films prepared at the substrate temperatures T (“C) shown. For clarity only the lower points on each plot are shown.
OPTICAL ABSORPTION IN GLOW DISCHARGE AMORPHOUS
Si FILMS
399
4. INFRARED ABSORPTION BANDS Glow discharge amorphous silicon contains several atomic per cent of hydrogen by virtue of its growth conditions. This improves the effectiveness of normal substitutional doping in controlling the electrical conductivity by passivating dangling silicon bonds. If high silane pressures and low substrate temperatures are selected, then it is possible to produce films closer to a silane polymer (SiH,), than to amorphous silicon containing hydrogen3sg. The less cohesive higher polymer films are not suitable for solid state devices. The IR absorption spectrum from 3 to 20 urn contains evidence of Si-H, Si-Si and possible contaminant groups. The magnitude of each absorption band can be used to estimate the numbers of each bond type in the sample. Brodsky et aL” have assigned the bands at approximately 2000-2100 cm-’ to SiH, stretch modes, those at 850-890 cm- 1 to SiH, bending (“deformation”) modes and that at 640 cm- ’ to SiH, wagging modes. From these it is possible to see how much of the hydrogen is incorporated as SiH, or SiH, rather than as SiH. In particular, the bending mode is absent for films with only SiH bonds. A typical survey IR spectrum from a Perkin-Elmer 580 double beam spectrophotometer, with an uncoated silicon substrate as reference, is shown in Fig. 3 for a film on crystalline silicon.
4000
3000 wavenumber
2000 cm”
1000
Fig. 3. The IR transmittance of an amorphous silicon film on a single-crystal silicon substrate (substrate temperature, 186 “C; Pirani gauge reading, 0.3 Torr). Each absorption band has detailed structure according to the number of hydrogen atoms bonded to each silicon atom.
The simple SiH stretching mode at 2000 cm-’ (ols) was only seen for high substrate temperature films immediately after their preparation. All films absorbed
400
J. MCGILL, J. I. B. WILSON
more strongly at the wavelengths assigned to SiH, and SiH, (a,‘, w3’) but the magnitude of the stretch mode absorption band decreased at the higher substrate temperatures (Fig. 4). The absorption coefficient for film 145 at 2090 cm-’ was films had much weaker approximately 1500 cm- l. The phosphine-doped absorption in this band than was expected from a comparison with undoped films prepared under similar conditions. The small band at 2265 cm-‘, apparent on all films, has not been identified. Figure 5 shows the bending mode absorption which was present for all our films. It was absent from high temperature IBM” and RCA7 films. Clear resolution into two bands (as shown for 147) occurred only for the lower temperature films, and this changed after a few days to a single broad band (as shown for 143). This may suggest growth of the SiH, groups at the expense of SiH,. Sample 145 had an absorption coefficient of approximately 1640 cm-r at 880 cm-‘.
147 146
800
1600 wavenumber
cm-’
900 wavenumber
600 -1
cm
Fig. 4. The SiH, stretch mode absorption band for films prepared at the substrate temperatures T ((C) shown and at a pressure of0.2-0.3 Torr. (The wavenumber resolution is better than 3 cm- ‘.) Samples 141 and 143 were doped. Fig. 5. The SiH, bending mode absorption resolution is better than 4 cm- I.)
band for the same films as in Fig. 4. (The wavenumber
OPTICAL
ABSORPTION
IN GLOW DISCHARGE
AMORPHOUS
Si FILMS
401
The strong wagging mode absorption at 640 cm-’ was weakest in the higher substrate temperature films and in the doped films. From assignments of the detailed structure of this band” our films again showed stronger absorption by SiH, and SiH, than by SiH. Only slight evidence of the 590 cm- 1 rocking mode was seen in all films. Aging of the films by’vacuum heating for 1 h at 200 “C (i.e. below the deposition temperature) produced a small increase in the bend and wagging mode absorptions but no change in the other absorption band. Zanzucchi et al.’ observed a similar apparent increase in hydrogen content after brief vacuum annealing. This part of the study is continuing and we have no explanation for the observations as yet. From the magnitude of the absorption bands we estimated that our films contained 3 x 1O22cm-3 Si-H bonds. 5.
CONCLUSIONS
The differences in geometry, r.f. power input and unidentified parameters between various glow discharge chambers make it difficult to define uniquely the gas pressure and substrate temperature required to produce good quality amorphous Si: H films. Although it was expected that temperatures around 250 “C and silane pressures around 0.1 Torr would produce films containing mainly Si-H, our apparatus gave films with a large proportion of SiH, and SiH, as evidenced by the presence of the bending mode absorption. The shape of the SiH, stretch mode was affected by the growth conditions and by heavy doping with phosphine. We have extended our study to correlate solar cell performance with film structure and we have already found that good photovoltaic diodes can be fabricated on our material. ACKNOWLEDGMENTS
We wish to thank Fiona Riddoch and Alec Wallace for assistance in film preparation. We are grateful to the SRC and the CEC for financial support of the project. REFERENCES
1 2 3 4 5 6 7 8 9 10
J. I. B. Wilson, J. McGill and S. Kinmond, Nature (London), 272(1978) 152-153. J. I. B. Wilson and J. McGill, Solid-State Electron Devices, 2 (1978) S7-SIO. M. H. Brodsky, Thin Solid Films, 40 (1977) L23-L25. R. C. Chittick, J. H. Alexander and H. F. Sterling, J. Electrochem. Sot., 116 (1969) 77. W. E. Spear and P. G. LeComber, Philos. Mug., 33 (1976) 935-949. J. Soohoo and R. D. Henry, J. Appl. Phys., 49 (1978) 801-803. P. J. Zanzucchi, C. R. Wronski and D. E. Carlson, J. Appl. Phys., 48 (1977) 5227-5236. R. J. Loveland, W. E. Spear and A. Al-Sharbaty, J. Non-Cryst. Solids, I3 (1973174) 55-68. M. H. Brodsky and M. Cardona, Local order as determined by electronic and vibrational spectroscopy: amorphous semiconductors, J. Non-Cryst. So/ids, 31(1978) 81-108. M. H. Brodsky, M. Cardona and J. J. Cuomo, Phys. Rev., Sect. B, I6 (1977) 3556-3571.